Cell polarity orients cytoskeletal filaments toward spatial landmarks, directing local cell growth. Protein kinases play key roles in cell polarity by translating spatial signals into regulation of cytoskeletal proteins including actin and myosin. At cell division, the actomyosin machinery dramatically reorganizes into a cytokinetic ring that constricts to separate the dividing cells. We hypothesize that cell polarity kinases act directly on the contractile actomyosin ring (CAR) to promote its assembly, organization, and function in cytokinesis. In this model, cell polarity signaling proteins regulate the CAR in cytokinesis analogous to their roles on other actomyosin structures in polarized growth. We study cell polarity and cytokinesis using the fission yeast S. pombe as a model system because it offers a well-defined parts list and a host of experimental advantages. Our preliminary data show that three conserved cell polarity protein kinases?Pak1, Kin1, and Pom1?directly phosphorylate CAR proteins and regulate specific steps in CAR assembly and constriction during cell division. In addition, regulators of the phosphatase PP2A are required to keep the CAR anchored in the correct place during cytokinesis. We propose to understand the molecular mechanisms by which these conserved protein kinases and phosphatases act on the CAR to promote cell division. The specific aims of this grant are to: (1) determine how Pak1 regulates the cytoskeleton during cell division, (2) test the hypothesis that Pom1, Kin1, and Pak1 represent sequential regulatory inputs for cytokinesis proteins, and (3) investigate how regulation of PP2A phosphatase positions the CAR. Successful completion of these goals will define a signaling system that prevents defects in cell division, and will identify previously unknown connections between two fundamental cellular processes: cell polarity and cytokinesis.